Portable chemical detector systems are required for the detection of explosives and other hazardous material. Such systems may be based on separation by gas chromatography followed by detection using a mass spectrometer, or on ion mobility spectrometry, or on mass spectrometry alone. Because the ambient concentration of the target analyte of interest is vanishingly low, other devices are often incorporated to improve the limit of detection. One such device is a chemical pre-concentrator, a device for boosting the concentration of an analyte of interest in a stream prior to analysis by a detector.
Exemplary components of a known pre-concentrator system are shown in FIG. 1. The pre-concentrator element itself is in essence a trap that will preferentially sorb a dilute analyte from a gas or liquid stream. Within the context of the present invention a sorbent material is one that will sorb a sample from a fluid—be that in the liquid or gaseous phase. To sorb is to take up a liquid or a gas either by adsorption or by absorption. Sorption is often enhanced by the use of a porous material or a chemically reactive layer of material. Examples of the former are carbon granules and sol-gel glasses, and examples of the latter are functionalised polymers. This material 101 is held on a mechanical support 102, which can be heated. Usually heating is carried out electrically, in that the passing of a current through the support 102 provides a corresponding heating of the support 102.
The trap is placed in a small enclosure 103 between three valves. The first valve 104 connects to a gas flow input 105, and the second valve 106 connects to a gas flow output 107. The third valve 108 connects to a subsequent analysis system 109. Pre-concentration typically involves a repetition of sorption and desorption steps.
FIG. 2 shows an example of a sorption step. The input and output gas flow valves 201 and 202 are opened, and the valve 203 connecting to the analysis system is closed. A gas stream 204 containing a small fraction of the target analyte 205 together with a large fraction of other gas molecules 206 is allowed to pass over or through the trap. Most of the analyte 207 is sorbed on the trapping layer 208, while the remainder of the gas stream is vented as exhaust 209.
FIG. 3 shows a corresponding desorption step. The input and output gas flow valves 301 and 302 are closed, and the connecting valve 303 is opened. The sorbed molecules are desorbed, usually by rapidly raising the temperature of the chemically sensitive layer 304 using the heater 305, and a concentrated flux of the analyte of interest 306 is passed into the analysis system or detector 307.
FIG. 4 shows in schematic form a detector system incorporating a pre-concentrator. Such systems including macroscopic pre-concentrators are available commercially. Pre-concentrator performance is defined in terms of the efficiency (i.e., the fraction of the desired analyte that is retained) and of the concentration factor (i.e. the increase in the desired analyte concentration). To maximize the efficiency, the surface area of the trap should be large as possible, and the sensitised coating highly attractive to the desired analyte, while to maximize the concentration factor, dead volumes should be as small as possible.
To reduce cycle times, the heated element should have low thermal mass. However, to increase the concentration factor even further without increasing the time needed for desorption, pre-concentrators can be used in a cascade consisting of a first trap with a large volume followed by a second trap with a small volume. The first trap has high efficiency but a long desorption time while the second trap has a short desorption time. Pre-concentrators containing even more stages are constructed in an analogous way.
The above considerations suggest that pre-concentrators are ideal candidates for miniaturization, and small traps based on capillaries were developed in the 1990s (Mitra and Yun 1993; Feng and Mitra 1998; U.S. Pat. No. 6,112,602). Increased integration with other components such as valves and gas chromatographs can be achieved by planar processing, and several planar pre-concentrators with thin-film heaters have been developed (U.S. Pat. Nos. 5,481,110; 6,171,378). Micromachined heaters with deep, etched trays filled with sorbent granules have also been demonstrated (Tian et al. 2003; U.S. Pat. No. 6,914,220). A flow-though pre-concentrator based on a sorbent polymer coating on a perforated heater has also been developed (US 20050095722). None of these configurations is entirely suitable for a compact system, since the valves needed for overall operation are often added by hybrid integration, causing an increase in dead volume and a reduction in concentration factor. A planar micro-machined valve and thermal desorber is described for use as a pre-concentrator with the advantages of low dead volumes and a high concentration factor (Syms and Yeatman; GB 2434643A). However, as described this device may suffer from a limited flow rate of sample from the ambient air during the sorption cycle. Furthermore, all of these devices are designed to be permanently coupled to the detector system, compelling the user to carry the entire system to any location of interest. This is not always feasible or practical.
Detector systems featuring a single-stage pre-concentrator that is also detachable from the detector are known. In some Concepts of Operations (CONOPS), it may not be possible take the detector system to the sample, and instead the detachable pre-concentrator may be hand-carried to a remote location and used to collect sample. Species of interest are gathered by a sorbent material in the pre-concentrator, and trapped. Once the sufficient sample has been collected remotely, the detachable pre-concentrator may be returned to the detector and then coupled with the detector, whereon the species of interest is desorbed and transferred to the detector system for analysis. A system of this type has been developed (Barket; Patterson; Gregory 2004, WO2006062906) and is commercially available.
However, the hand-portable sample collection devices of the type disclosed have the disadvantage of being relatively expensive, bulky units which typically include pumps, sorbent tubes, valves and flow meters. The size and cost of these units limits their deployability. For example, a sample collection device with a weight of four pounds is excessive and cannot be given to every soldier unless it is at the expense of other equipment.
More importantly, for the sample collector disclosed in WO2006062906 and similar single stage pre-concentrators, there are difficulties in efficiently transferring the collected sample to the preferred analytical system, a gas chromatography mass spectrometer (GC-MS). These difficulties may increase the technical complexity of the analysis, increase the duration of the analysis, and lead to loss of potentially valuable sample.
Firstly, the flow of gas required to efficiently flush the analyte from the pre-concentrator during the desorption step may be much higher than can be directly accommodated by the GC column. When this is the case, the excess flow (containing valuable analyte) is discarded through the use of a splitter, thereby reducing the overall sensitivity. Alternatively, the pre-concentrator can be allowed to discharge through a conventional sample loop. However, this is again very wasteful, as the injected volume (typically 1-2 ml) will be a small fraction of the total volume of gas used to flush the pre-concentrator.
Secondly, the use of conventional pipe-work, unions, valves and other gas handling hardware introduces considerable dead volume into the system. During the desorption step, the analyte is therefore discharged into a much larger volume of carrier gas than might otherwise be desirable.
Thirdly, a typical GC system requires that the temporal profile of the injected analyte be of the order of a few seconds wide. When using direct injection, the mass of a conventional pre-concentrator is such that the sorbent cannot be heated fast enough to achieve such a narrow desorption profile, and as a result, a secondary refocusing step is generally required. In the case of involatile analytes, refocusing is achieved by trapping at the head of the column by using a suitably low column temperature whereas volatile analyte must be refocused using a cryotrap. Clearly, the provision of cryogenic materials in the field or to a portable GC is highly inconvenient.
There is therefore a need for improved detection systems.